linux-kernel - Re: [RFC 4/4] cputime: remove scaling

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Message-ID: <11f12774-5353-4380-82d0-e22707e11729@email.android.com>
Date:	Wed, 10 Apr 2013 07:29:21 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Stanislaw Gruszka <sgruszka@...hat.com>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, rostedt@...dmis.org,
	akpm@...ux-foundation.org, tglx@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 4/4] cputime: remove scaling

I have a patch that does scaling by multiply for 64-bit architectures.  I probably should clean it up and send it in.  I need to see if it fixes this problem.

Ingo Molnar <mingo@...nel.org> wrote:

>
>* Stanislaw Gruszka <sgruszka@...hat.com> wrote:
>
>> Scaling cputime cause problems, bunch of them was fixed, but still is
>possible 
>> to hit multiplication overflow issue, which make {u,s}time values
>incorrect. 
>> This problem has no good solution in kernel.
>
>Wasn't 128-bit math a solution to the overflow problems? 128-bit math
>isn't nice, 
>but at least for multiplication it's defensible.
>
>> This patch remove scaling code and export raw values of {u,t}ime .
>Procps 
>> programs can use newly introduced sum_exec_runtime to find out
>precisely 
>> calculated process cpu time and scale utime, stime values
>accordingly.
>> 
>> Unfortunately times(2) syscall has no such option.
>> 
>> This change affect kernels compiled without
>CONFIG_VIRT_CPU_ACCOUNTING_*.
>
>So, the concern here is that 'top hiding' code can now hide again. It's
>also that 
>we are not really solving the problem, we are pushing it to user-space
>- which in 
>the best case gets updated to solve the problem in some similar fashion
>- and in 
>the worst case does not get updated or does it in a buggy way.
>
>So while user-space has it a bit easier because it can do floating
>point math, is 
>there really no workable solution to the current kernel side integer
>overflow bug? 
>I really prefer robust kernel side accounting/instrumentation.
>
>Thanks,
>
>	Ingo

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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